The aim of these studies is to describe the evolution of the simian virus 40 (SV40) variants which arise in rhesus monkey kidney cell cultures persistently infected with this virus. At various times after initial infection with standard SV40 we will characterize the virions with respect to biological and physical properties that might relate to the establishment and maintenance of the resultant persistent infection. The defective interfering particle, cell killing particle, and transforming particle activities of the carrier culture virions will be titered and related to the plaque forming activities. The profiles of these activities in CsCl equilibrium density gradients will be compared in order to determine which activities are separable. The DNA of carrier culture virions will be analyzed to determine whether there are virions with short DNA molecules, with cellular sequences, and with reiterated sequences of viral or cellular origin. The analysis of carrier culture virus DNA will also be carried out with samples taken from different regions of the equilibrium density gradient profiles in order to establish correlations between the biological and physical characteristics of these virions. The defective interfering particle and cell killing particle activities will be assayed by determining the dose response curves for yield reduction and single cell survival, respectively. Transforming activity will be titered by agar suspension culture for titration of transformed cells. DNA will be analyzed by velocity gradient centrifugation, gel-electrophoresis, and DNA-DNA hybridization. DNA will also be analyzed by restriction endonuclease digestion and electrophoresis of resultant fragments.